Method of creating colored materials by fixing ordered structures of magnetite nanoparticles within a solid media

ABSTRACT

Compositions and methods wherein ordered structures of photonic nanocrystals are created in a liquid medium and then such structures are fixed by converting the liquid medium to a solid. In addition, compositions and methods of reversibly fixing such structures, so that ordered structures can be reversibly created in a liquid medium, converted to solid, and then converted back to liquid, wherein new ordered structures can be created and again fixed.

FIELD OF THE INVENTION

This invention relates to methods of creating colored materials byfixing ordered structures of magnetite nanoparticles within a solidmedia, such that the ordered structures diffract light to create colors.

BACKGROUND

Superparamagnetic nanocrystals, or photonic crystals, which are capableof forming ordered structures that diffract light to create colors, havebeen previously described. For example, Yin et al, SuperparamagneticMagnetite Colloidal Nanocrystal Clusters, Angwantde Chemie, 46:4342(2007), Magnetically responsive colloidal photonic crystals, Journal ofMaterial Chemistry 18: 5041 (2008), Self-Assembly and Field-ResponsiveOptical Diffractions of Superparamagnetic Colloids, Langmuir 24:3671(2008), Assembly of Magnetically Tunable Photonic Crystals in NonpolarSolvents, JACS 131: 3484 (2009), and WO2009/017525, all incorporatedherein by reference, all describe the synthesis of magnetitenanocrystals, or photonic crystals, which can be induced to form orderedstructures when exposed to a magnetic field. Furthermore, these orderedstructures can be tuned by varying the strength of the magnetic fieldsuch that different diffractive patterns and colors are created. Howeverthese previous efforts have required a constant magnetic field in orderto maintain the ordered structure and thus the color.

Accordingly, it would be desirable to have materials and methodscomprising photonic nanocrystals which can be tuned in a liquid mediumto create ordered structures which impart color, and which such orderedstructures can be fixed by converting the liquid medium to solid. Itwould further be desirable to create ordered structures of photoniccrystals in a medium which can be reversibly converted from liquid tosolid, such that the color can be changed.

SUMMARY

In its broadest scope, the invention described herein comprisescompositions and methods wherein ordered structures of photonicnanocrystals are created in a liquid medium and then such structures arefixed by converting the liquid medium to a solid. Further provided aremethods of reversibly fixing such structures, so that ordered structurescan be reversibly created in a liquid medium, converted to solid, andthen converted back to liquid, wherein new ordered structures can becreated and again fixed.

In accordance with an exemplary embodiment, a method of creating coloredmaterials, comprises: fixing ordered structures of magnetitenanoparticles within a media, such that the ordered structures diffractlight to create colors.

In accordance with another exemplary embodiment, a method of generatingmulticolored patterns comprises: fixing a structural color from asuperparamagnetic collidal nanocrystal clusters (CNC or CNCs); andintroducing a high resolution patterning of multiple structural colorsusing a single material.

In accordance with a further exemplary embodiment, a full color printingand particle encoding based on artificial structural colors from amagnetically tunable photonic crystal, the printing and particleencoding comprises: a plurality of magnetite nanoparticles; ethanol; anda photocurable resin.

In accordance with another exemplary embodiment, a method of formingmagnetochromatic microspheres comprises: coating a plurality ofmagnetite nanocrystals with a medium; dispersing the plurality of coatedmagnetite nanocrystals in a curable solution; placing the magnetitenanocrystals and curable solution in an immiscible solution to form anemulsion; exposing the emulsion to an external magnetic field, whichaligns the coated magnetite nanocrystals in one-dimensional chainswithin emulsion droplets within the curable solution; and curing theemulsion droplets within the curable solution into magnetochromaticmicrospheres.

In accordance with a further exemplary embodiment, a magnetochromaticcomposition formed by the method as recited above, and wherein thecomposition is used for a color display, signage, bio and chemicaldetection and/or magnetic field sensing.

In accordance with another exemplary embodiment, a method of formingmagnetochromatic microspheres comprises: a simultaneous magneticassembly and UV curing process of an emulsion system comprised ofsuperparamagnetic Fe₃O₄@SiO₂ colloidal particles, which areself-organized into ordered structures inside emulsion droplets of UVcurable resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a schematic illustration of the mechanism for generatingmultiple structural colours with a single material, wherein FIG. 1( a)shows the main concept of immobilization of structure of CNCs inphotocurable resin having a superparamagnetic core and ethanol solvationlayer allows the stable dispersion of the CNCs in the liquid resin, andupon the application of an external magnetic field, CNCs are assembledto form chain-like photonic crystal, and UV exposure instantaneouslyfixes the ordered structure in polymeric matrix.

FIGS. 1( b)-1(g) are schematic illustrations of the multicolourpatterning of structural colour with single material by a sequentialaction of “tuning and fixing”, and wherein the diffraction wavelength istuned by varying the strength of magnetic fields, and spatiallypatterned UV light polymerizes the photocurable resin and fixes theposition of ordered CNCs; and wherein after polymerization, remnantliquid resin is washed away with unreacted PEG-DA monomer solution; andwherein FIG. 1( h) shows the mechanism for creation of various coloursfrom a single ink; and wherein the UV curing of the M-ink under magneticfields with different strengths can freeze the chain-like assemblieswith different inter-particle distances which determine the diffractedwavelength of light: shorter diffracted wavelength for shorterinterparticle distance.

FIG. 2( a) is a reflection micrograph of multicoloured structural colourgenerated by gradually increasing magnetic fields, and wherein themicrostructure (i) is generated under no magnetic field, andmicrostructures (ii) to (viii) are generated under gradually increasingstrength of magnetic field from 130 G to 700 G.

FIG. 2( b) is a transmission micrograph of the same sample of FIGS. 2(a) and 2(c), and the corresponding spectra of the microstructures, andwherein the microstructure (i) does not show any diffraction peak in thevisible range, and wherein the microstructures (ii) to (viii) show theshift of the diffraction peak to the shorter wavelength, and wherein thescale bars is as follows: 100 μm in FIGS. 2( a), 2(b), 2(e), 2(f); 1 μmin FIG. 2( d), and 250 μm in FIGS. 2( g)-2(i).

FIG. 2( d) is an SEM image of the sliced cross section of a photocuredsample, and wherein the dimpled surface profile shows the traces ofchain-like ordering of CNCs.

FIG. 2( e) is an SEM image of concentric patterns of a triangle, asquare, a pentagon and a circle.

FIG. 2( f) is an SEM image of multicoloured barcodes.

FIG. 2( g) is an SEM image of a composite pattern of strip and polygon.

FIGS. 2( h) and 2(i) are reflection and transmission micrographs,respectively of a tree.

FIGS. 3( a)-3(g) show reflection intensity modulation and spatial colourmixing of structural colour, and having a scale bars as follows: 250 μmin FIGS. 3( a) and 3(d), and 100 μm in FIG. 3( g).

FIG. 3( a) is a 4-bit reflection intensity modulation by the varyingnumber of monotone structural colour dots, and wherein each of the reddotted lines stands for a pixel which shows distinct level of reflectionintensity.

FIG. 3( b) is a reflectance spectrum of the corresponding 16 pixels ofFIG. 3( a).

FIG. 3( c) is a monotone 4-bit image of Mona Lisa which consists of 4800pixels.

FIG. 3( d) is a spatial colour mixing of structural colour, wherein eachpixel of 4×4 matrix consists of different colour dots, and each of whichis size of approximately 25 μm.

FIG. 3( e) is a corresponding reflectance spectra of selected pixels inFIG. 3( d) (inset), and wherein the green line in the spectra stands forthe spectrum of (1,1) component of the pixel, orange line for the (1,2)component, gray line for the mathematical addition of the green andorange line, and blue line for the normalized spectrum of full pixel.

FIG. 3( f) is a reproduction of butterfly, Papilo Palinurus, and whereinthe colour of wings in the reproduced image shows structural colourmixing by mixing blue and yellow-green.

FIG. 3( g) is a magnification of wing area of FIG. 3( f), which consistsof blue and yellow-green dots, and wherein each dot is the size of16.7×16.7 μm² (˜1500 DPI).

FIGS. 4( a)-4(f) show colour and shape encoded particles fabricated inmicrofluidic environment using a single ink, wherein FIGS. 4( a)-4(c)are schematic diagrams for generating encoded particles using M-Ink inPDMS microfluidic channels; FIG. 4( d) is a free floating encodedparticle with various shape and colour around the PDMS anchor area; FIG.4( e) is an enlarged micrograph of FIG. 4( d), showing closely packedparticles with various colour and shape; and FIG. 4( f) areheterogeneously encoded particles embedded with small colour dots. Scalebars: 200 μm in FIG. 4( d)-4(f).

FIG. 5( a) is a schematic of a synthetic procedure for themagnetochromatic microspheres, where when dispersed as emulsiondroplets, superparamagnetic Fe₃O₄@SiO₂ core-shell particlesself-organize under the balanced interaction of repulsive and attractiveforces to form one-dimensional chains, each of which containsperiodically arranged particles diffracting visible light and displayingfield-tunable colors, and UV initiated polymerization of the oligomersin emulsion droplets fixes the periodic structures inside themicrospheres and retains the diffraction property.

FIG. 5( b) is an SEM image of Fe₃O₄@SiO₂ particle chains embedded in aPEGDA matrix.

FIG. 5( c) are schematic illustrations and optical microscopy images forthe magnetochromatic effect caused by rotating the chain-like photonicstructures in magnetic fields.

FIG. 6( a) is a schematic illustration of the experimental setup forstudying the angular dependence of the diffraction property of themagnetochromatic microspheres.

FIG. 6( b) is a reflection spectrum and corresponding digital photorecorded from a single Fe₃O₄@SiO₂/PEGDA microsphere at different tiltingangles.

FIGS. 7( a)-7(f) are optical microscopy images (500×) ofmagnetochromatic microspheres with diffractions switched between “on”(a, c, e) and “off” (b, d, f) states by using external magnetic fields,and wherein these microspheres were prepared using (a, b) 127, (c, d)154, and (e, f) 197 nm Fe₃O₄@SiO₂ colloids.

FIG. 8( a) are dark-field optical microscopy images of a series ofFe₃O₄@SiO₂/PEGDA microspheres with diameters from approximately 150 μmto 4 μm, and wherein the larger microspheres were fabricated in mineraloil and smaller ones in silicon oil.

FIGS. 8( b)-8(d) are top view to side view SEM images of themicrospheres, showing some of the Fe₃O₄@SiO₂ particle chains aligned onthe surface along the longitudinal direction, and wherein it should benoted that a plurality of particle chains are embedded inside themicrospheres, with only ends occasionally observable in the top viewimage (b).

FIGS. 9( a)-9(b) are statistical diagrams showing the turning thresholdof field strength for Fe₃O₄@SiO₂/PEGDA microspheres with differentloadings of magnetic particles, wherein FIG. 9( a) is 8 and FIG. 9( b)is 6 mg Fe₃O₄/ml PEGDA, and wherein the diagrams show the percentage ofviewable area which is turned on at certain field strengths, and thecorresponding accumulative curves.

FIGS. 10( a)-10(d) are schematic diagrams of the optical response ofFe₃O₄@SiO₂/PEGDA microspheres in a (a, b) 1.22 and (c, d) 3.33 Hzvertical/horizontal alternating magnetic field, wherein Hs/Ho is theratio of reflection with H field to that without H field.

FIGS. 11( a)-11(i) are digital photos and reflection spectra of threetypes of Fe₃O₄@SiO₂/PEGDA microspheres loaded in 1.8×1.8×0.1 cm glasscells filled with PEG (Mw=1500), wherein the diffraction is switched on(a, d, g) or off (b, e, h) by melting the PEG matrix, rotating themicrospheres with a magnetic field, and finally cooling down the PEGmatrix to lock the sphere orientation such that bistable states cantherefore be maintained in the absence of magnetic fields, and thecorresponding reflection spectra (c, f, i) display diffraction peaks atthe “on” stage and none at the “off” stage.

DETAILED DESCRIPTION

The invention described herein provides various methods of fixing theordered structure by (1) using an external magnetic field to createordered structures of photonic crystals in a liquid medium, and (2)converting the liquid medium to a solid medium to preserve the orderedstructure, such that it remains when the external magnetic field isremoved.

In accordance with an exemplary embodiment, the media (or medium) of theinvention can be any media or medium capable of phase change from aliquid to a solid phase. Transparent, semi-transparent, or translucentmedium is preferred. Exemplary media include, but are not limited to UVcurable resins, such as polyethyleneglycol diacrylate (PEGDA) oligomersin combination with trace amount of photo initiator2,2-Dimethoxy-2-phenylacetophenone (DMPA), acrylic, epoxy, polyester,stereolithography resins, or other liquid media capable of beingconverted to a solid upon exposure to UV light. The media of theinvention further comprise light-curable, temperature-curable,air-curable, and energy-curable liquid media capable of being convertedto solid form. In accordance with an exemplary embodiment, the inventionfurther comprises media which can be reversibly converted from liquid tosolid and back to liquid, such as that described in “CARIVERSE resin: athermally reversible network polymer for electronic applications” Chang,et al, Electronic Components and Technology Conference, 1999. 1999Proceedings. 49^(th) Volume, Issue, 1999 Page(s):49-55 hereinincorporated by reference, Polyethelene glycol films (polyethyleneglycol films), and/or paraffin.

In accordance with an exemplary embodiment, the media (or medium) of theinvention can comprise a film, beads, microspheres, and any3-dimensional shape which is desired.

The invention consists of ordering the photonic crystals within themedia (or medium) using an external magnetic field to attain a desiredspacing which will create a desirable color by diffracting light, andthen subjecting the medium to conditions which cause it to convert to asolid, which solidifies and fixes the photonic crystals in the orderedstructure such that the color is preserved.

In some embodiments, the solidification of the media (or medium) resultsis done in bulk, in other embodiments the solidification is performed onvery small scales to create and fix local regions of color, creatingfine features and the ability to create multi-colored patterns.

Provided herein are two exemplary embodiments of the invention. Thefirst is a method of creating detailed multicolored patterns by localtuning and fixing of ordered structures. The second is a method ofcreating microspheres containing fixed ordered structures. Furtherprovided is a method of creating a display using ordered structurecontaining microspheres.

Full Colour Printing and Particle Encoding Based on ArtificialStructural Colours from a Magnetically Tunable Photonic Crystal

It can be appreciated that many creatures in nature, such asbutterflies, beetles, and peacocks display unique iridescent andmetallic colors (or colours), known as “structural colors” or“structural colours”, which result from the light interaction withperiodic nanostructures on their surface¹⁻⁶. Without relying on multiplepigments or dyes, various colored patterns are cleverly produced using asingle structural material by simply altering the dimension of thenanostructures. Cost-effective and scalable implementation of thisfeature in manufacturing would greatly simplify production ofmulticolored goods such as electronics, displays, and vehicles. Therehave been many attempts to produce artificial structural coloredpatterns using various bottom-up and top-down techniques in a variety ofresearch fields⁷⁻²². However, mimicking such nanostructures found in thenatural world requires state-of-the-art nanofabrication techniques thatare expensive and not scalable. Especially, production of multi-colorsand patterning of such structure were not possible with a singlestructural material.

A high resolution patterning and artificial production of multiplestructural colors based on successive tuning and fixing the structuralcolor of a single structural material is demonstrated in accordance withan exemplary embodiment. In accordance with another exemplaryembodiment, a color tunable structural material, whose color ismagnetically tunable and lithographically fixable is disclosed. Usingphotonic crystals, a curable resin, and a specially designedlithographic instrument, fine nanostructures for scalable production ofa structural color can be generated, tuned the color through the entirevisible spectrum by magnetically changing the dimension of thenanostructures, and immobilized the nanostructures lithographically toproduce patterns with arbitrary spatial arrangements of color. Inaddition, in accordance with a further embodiment, two applications ofthe disclosed system including high resolution color patterning forprinting and micro-scale particle encoding for bio-assays aredemonstrated. With the superior scalability and simplicity, themulticolor production scheme as disclosed herein can have a significantimpact on the color production for both special instruments and generalconsumer goods.

It can be appreciated that structural colors in nature such as butterflywings, beetle cuticles and peacock feathers have attracted considerableattention in diverse research areas¹⁻⁶. Structural color shows manycharacteristics different from chemical pigments or dyes. For example,as can be found in the feathers of a peacock, various colors result fromthe interaction of light with a single biological material, melaninrods, and its iridescent colors can be determined by the lattice spacingof the rods⁵. In nature, a single biological material with differentphysical configurations displays various colors and it greatlysimplifies the manufacturing process to produce multiple colors. Theunique colors originating from the physical structures are iridescentand metallic, and cannot be mimicked by chemical dyes or pigments. Also,structural color is free from photobleaching unlike traditional pigmentsor dyes.

Due to its unique characteristics, there have been many attempts to makeartificial structural color through various technological approachessuch as colloidal crystallization⁷⁻¹⁸, dielectric layer stacking^(19,20)and direct lithographic pattering^(21,22). Colloidal crystallizationtechnique is most frequently employed to make a photonic crystal, whichblocks a specific wavelength of light in the crystal and thereforedisplays the corresponding color. Gravitational force⁷, centrifugalforce⁸, hydrodynamic flow⁹, electrophoretic deposition¹⁰ and capillaryforce from the evaporation of solvents¹¹⁻¹⁸ are utilized to assemble thecolloidal crystals. Although these methods produce structural colorswith large-area, the growth of colloidal crystals usually takes a longtime for better crystallization and fewer defects. Also, since the bandgap of a photonic crystal is dependent on the size of colloids,different sizes of colloidal suspensions are needed to producemulticolored structures. Furthermore, there have been greattechnological difficulties in assembling colloids of different sizes toform multicolored patterns with fine resolutions.

Dielectric layer stacking and lithographic pattering of periodicdielectric material generate structural color by directly controllingthe submicrometer structure of the surface. Diverse fabricationprocesses were reported such as replicating natural substrates¹⁹,depositing materials layer by layer²⁰ and etching substrate usingvarious lithographic techniques^(21,22). These approaches areadvantageous in that they accurately fabricate periodic dielectricstructure on the surface, which controls the desired photonic band gap.However, in spite of the advantage of sculpting sophisticatednanostructures in a well controlled manner, a cost-effectivemanufacturing scheme to generate multicolored structures over a largearea is hard to achieve due to the requirement of a vacuum process.Moreover, great effort is necessary to produce multicolored patterns ona substrate since different pitches of dielectric stacks are requiredfor different colors.

Recently, dynamic tuning of structural color with a single material hasbeen demonstrated by exerting an external magnetic field on a solutionof photonic crystals. This magnetically tunable photonic crystal showsbroad tunability in its photonic band gap covering the whole visiblespectrum and has a fast response time^(23,24). However, the color ofthis material cannot be fixed permanently because the external magneticfield is required to maintain structural order. If one couldinstantaneously ‘freeze’ the photonic crystal structure of the photoniccrystals with great spatial resolution, the artificial patterning ofvarious structural colors with a single material would be possible.

In accordance with an exemplary embodiment, an instantaneous fixing ofstructural color from photonic crystals and introduce high resolutionpatterning of multiple structural colors using a single material, isdescribed herein. Both material system and special instrumentation aredeveloped to overcome the limitations of the previous approaches toproduce artificial structural colors. In accordance with an exemplaryembodiment, the applications of this promising technology: structuralcolor printing for design materials and structural color encodedparticles for biochemical assay are disclosed.

It can be appreciated that it would desirable to generate multicoloredpatterns with high resolution using a single material by repetitivetuning and fixing the structural color from the mixture ofsuperparamagnetic photonic crystals and photocurable resin (FIG. 1). Inaccordance with an exemplary embodiment, the superparamagnetic photoniccrystals, each consisting of many single domain magnetite nanoparticles,which are capped in a shells, which is preferably a silica shell²⁴. Itcan be appreciated that the superparamagnetic photonic crystals are anycomposition which can form ordered structures when exposed to anexternal magnetic field, such that the ordered structures diffract lightto create color. preferably the photonic crystals are composed ofmagnetite (Fe₃O₄). In addition, the magnetite nanoparticles can becoated in shells of other suitable mediums, including but not limited tosilica, titania (titanium oxide), and/or polymers such as polystyreneand polymethylmethacrylate. The coating process provides a means toobtain good dispersibility and promotes solvation repulsion in thephotocurable solution or resin. The polymers such as polystyrene andpolymethylmethacrylate can be used after a necessary surfacemodification. In accordance with an exemplary embodiment, the thicknessof the silica coating can be controlled by controlling the amount ofsilane precursors or the catalyst. The thickness control can be found in(1) Ge, J. and Yin. Y., “Magnetically Tunable Colloidal PhotonicStructures in Alkanol Solutions”, Adv. Mater., 2008, 20, 3485-3491. (2)Yin, Y.; Lu, Y.; Sun, Y. and Xia, Y., “Silver Nanowires Can Be DirectlyCoated with Amorphous Silica to Generate Well-Controlled CoaxialNanocables of Silver/Silica”, Nano Lett. 2002, 2, 427-430, both hereinincorporated by reference.

It can be appreciated that in accordance with an exemplary embodiment,in solution, the magnetite particles are attracted to each other andwill aggregate unless treated to create balancing repulsive forces. Suchbalancing forces can be created by solvating the particles in a solutionwith a positive charge, which will repel neighboring positively chargedparticles. Alkanols, ethanol, and other solvation solvents can be usedfor this function. Alternatively, coatings can be applied to theparticles to create optimal repulsive forces to balance the attractionthe magnetite particles will have for each other. For example, thecompositions and methods described in U.S. Provisional PatentApplication Ser. No. 61/154,717, “Assembly of magnetically tunablephotonic crystals in nonpolar solvents,” herein incorporated byreference, can be employed to produce particles with the proper balanceof attractive and repulsive forces.

Without an applied external magnetic field, the photonic crystals arerandomly dispersed in the photocurable resin and display a brown colorwhich is the intrinsic color of magnetite. Under the external magneticfield, the photonic crystals are assembled to form chain-like structuresalong the magnetic field lines^(25,26). Attractive magnetic force due tothe superparamagnetic core is balanced with repulsive solvation force,both of which determine the inter-particle distance. The inter-particledistance in a chain determines the color of the light diffracted fromthe chain, which can be explained by Bragg diffraction theory. Thus, thecolor can be tuned by simply varying the inter-particle distance usingexternal magnetic fields of varying strength. Once the desired color isobtained, it can be fixed by solidifying the photocurable resin throughUV exposure. The particle chains can be frozen in the solidified polymernetwork without distorting its periodic arrangements, thus retaining thestructural color.

However, the above fabrication scheme previously could not be achievedbecause of the difficulty of maintaining the tunability of photoniccrystals in photocurable resin and the instantaneous immobilization ofthe chain structure. A simple dispersion of CNCs in photocurable resindoes not possess strong and long-range repulsive inter-particle forcesthat can cooperate with the magnetically induced attractive force toallow dynamic tuning. Without a strong repulsion, the photonic crystalsirreversibly aggregate with each other when they are pushed togetherupon application of the external magnetic fields^(27,28). A stronghydrogen bonding solvent such as Alkanols can form a relatively thicksolvation layer around the particle surface which can provide strongrepulsion when two solvation layers overlap²⁴. In accordance with anexemplary embodiment, the problem of aggregation and dynamic assembly inphotocurable resin has been solved by adding a small amount of ethanolto the system. It can be appreciated that this three phase system,composed of photonic crystals, ethanol, and photocurable resin, cansuccessfully stabilize the photonic crystals and maintain the colortunability (FIG. 1( a)). Once the photonic structures are fixed, thegradual evaporation of ethanol will not disturb the structural color.

The second challenge was to develop a rapid solidification process toprevent distortion of photonic nanostructure²⁹. In accordance with anexemplary embodiment, a photopolymerization can be used to achievelithographic high resolution patterning of the photonic crystals. Incomparison to the other solidification methods such as thermal curing,photocuring is instantaneous and can rapidly fix the color of thephotonic crystals achieved by tuning the external magnetic field.Because of its instantaneous nature, photocuring also allows localizedsolidification for high resolution patterning by avoiding significantfree-radical diffusion during polymerization³⁰, making it possible touse techniques such as optofluidic maskless lithography (OFML)³¹ forcreating desired microscale patterns. Any UV or directed energy systemcapable of creating localized polymerization or curing of liquid mediato solid can be used. In accordance with an embodiment, poly(ethyleneglycol) diacrylate (PEG-DA or PEGDA) with a photoinitiator(2,2-dimethoxy-2-phenylacetophenone) can be used as the photocurableresin. Other suitable photocurable resins include ethoxylatedtrimethylolpropane triacrylate (ETPTA), PEG-DA of various molecularweights (Mw: 258, 575, 700), 2-hydroxyethyl methacrylate (HEMA),methylmethacrylate (MMA), acrylamide (AAm), allyamine (AM), and/or anycombination thereof.

In accordance with another exemplary embodiment, the instantaneousillumination of focused UV energy has been achieved by exploiting thepreviously reported OFML system, a versatile tool for dynamicallygenerating heterogeneous microstructures by in-situ photopolymerizationin microfluidic environment. Fast microelectromechanical system (MEMS)based spatial light modulator inside the system provides instantaneousillumination (less than (<) 80 ms) of patterned UV light to thephotocurable resin^(31,32). Using this system, the chain structure canbe preserved without distortion. Compared with the traditional methodsfor generating structural color by the slow growth of colloidal photoniccrystals, the magnetic assembly followed by photopolymerizedimmobilization can be accomplished within seconds with a high degree ofspatial control.

Various multicolored patterns can be generated with a single material bya sequential process involving cooperative actions of magnetic fieldmodulation and spatially controlled UV exposure (FIG. 1( b)-(g). Inaccordance with an exemplary embodiment, a PEG coated glass slide wasused as a substrate to avoid adhesion of the photonic crystals onto thesurface of a bare glass slide. A thin layer of photonic crystals incurable liquid resin is then deposited on the substrate (FIG. 1( b).Once a desired color of the photonic crystals is obtained by exerting amagnetic field, the patterned UV exposure fixes the color locally,producing a colored pattern at specific regions (FIG. 1( c)). Then, thecolor of uncured liquid resin is changed by simply varying the strengthof magnetic field. Subsequent controlled UV exposure produces anothercolored pattern in a different location (FIG. 1( d)). As illustrated byFIG. 1( b)-1(g), micropatterns with different structural colors (FIG. 1(h)) can be easily formed by repeating this “tuning and fixing” process.No movement of substrate is required for deposition of multiple inkmaterials since the photonic crystal solution is deposited only once atthe beginning of the process. Also multiple patterns can be exposedwithout movement of both substrate and mask since the OFML systemdynamically controls the pattern of multiple UV exposure without theneed of changing physical photomasks. Therefore, the methods asdescribed herein combine the advantages of photonic crystals and OFML,and can achieve high resolution heterogeneous patterning rapidly byeliminating the need for alignment and registration.

In order to demonstrate the concept of generating structural color witha single ink, multicolor structures were fabricated by the method asdescribed above.

The reflective optical microscope image (FIG. 2( a)) and thecorresponding spectrum data (FIG. 2( c)) of each microstructure showsgradual color changes from red to blue as the applied magnetic fieldstrength is gradually increased. This gradual increase in externalmagnetic field induces increasing attractive force between the inducedmagnetic dipole moment of photonic crystals, thereby decreasing theinter-particle distance in a chain. In agreement with the Braggdiffraction theory, the spectra blue shift as the result of the gradualdecrease in the inter-particle distance. It is worth noting that thistuning of the colors of photonic crystals does not suffer fromhysteresis and is very reproducible due to the paramagnetic nature ofphotonic crystals. Furthermore, the wide tuning range covering the wholevisible spectrum is owing to the strong magnetic attractive force fromthe superparamagnetic property of photonic crystals as well as therepulsive forces with comparable strength. In this case, the repulsionis composed of the relatively weak but long-range electrostatic forceand the relatively strong but short-range solvation force resulting fromthe ethanol solvation layer of the photonic crystals. Colors of thecorresponding microstructures shown in the transmission microscope (FIG.2( b)) are all brownish, the intrinsic color of magnetite, which arequite different from those of the reflective optical microscope image.This unique difference between the reflection image and the transmissionimage further proves the formation of structural color, whose colorationmechanism is not based on the absorption of light like typical pigmentsand dyes. Since the photonic crystal structure can be frozen within thepolymeric matrix, the chain structures directly were confirmed, whichusually de-assembles in solution after removal of the magnetic field. Asshown in FIG. 2( d), a scanning electron microscope (SEM) image of thesliced cross section with laser microtome of the cured resin revealsthat the diffraction of structural color does come from the periodicarrangement of the particles in the chain. The dimpled structures of thesliced cross sectional plane are the traces of the ordered photoniccrystals. Also, this shows that the photopolymerization by OFMLpreserves the original chain structure formed in the liquid phase.

By controlling the UV exposure pattern and magnetic field strength asdescribed in FIG. 1, it can be appreciated that a high resolutionpatterning of multiple structural colors with different geometries andcolors can be produced. FIG. 2( e) shows four different multicoloredpatterns, and each of them is fabricated with five concentric UVpatterns under various magnetic field intensities. Furthermore, barcodedmicrostructures composed of sixteen colorful strips are also fabricatedby sixteen sequential exposures (FIG. 2( f)). It can be appreciated thatthere is no alignment error since there is no movement of the substrateduring the exposure. The width of the bar code is only 10 μm which showshigh resolution spatial patterning of structural colors. Spatialpositioning of a smallest feature of structural color depends on thesize of diffracting unit and resolution of the lithography. Since thesize of CNCs (approximately 170 nm) is smaller than the resolution ofour optical system, the spatial positioning of the structural color ismainly determined by the resolution of the optical system, which can beenhanced up to the limit of typical optical lithographic resolutions³³.Colorful heterogeneous microstructures of any desired shape and colorare easily achieved as shown in FIGS. 2( g)-2(i).

For detailed depiction of an image, not only producing structural colorof single color depth as shown in FIG. 2, but also grayscale modulationand color mixing are required to broaden the ability of colorexpression. The proposed scheme of generating structural color caneasily be merged with well developed reprographic techniques such ashalftoning and dithering^(34,35), and broaden the capability of colorexpression. Current digital reprographic technique expresses grayscaleby varying density of dots in a pixel which is smaller than the humaneye's resolution. In accordance with an exemplary embodiment, analogousto traditional grayscale expression, the overall reflection intensitycan be modulated by the number of color dots, and present similargrayscale effects. For the proof-of-concept demonstration, 16 pixelarrays were generated, and each of them consists of 25 μm×25 μm dotswhose configuration is based on the Bayer pattern³⁴ (FIG. 3( a)).Reflection intensity shown in FIG. 3( b) verifies 16 distinct intensitylevels of corresponding pixel arrays. As an example for reflectionintensity modulation, as shown in FIG. 3( c) a 4-bit monotone image ofMona Lisa, a 16th century Italian portrait by Leonardo da Vinci wasreproduced. Reflection intensity of each pixel is modulated by varyingthe density of dots with 16 levels.

Besides the reflection intensity modulation, spatial color mixing can beachieved by parallel distribution of color dots. Quantized dot arrayscomposed of different colors can be seen as a single mixed color whentheir size is below human eye's resolution. To demonstrate spatial colormixing of the structural color, a 16 pixel arrays was fabricated (FIG.3( d)), and each pixel is composed of 16 dots of 2 or 3 differentcolors. Spectrum of color mixed pixel (FIG. 3( e)) shows that simplesummation of the two different color spectrum results in the totalreflection spectrum, proving the spatial mixing of the distinctstructural colors. It can be appreciated that this simple spatial mixingscheme of structural color exists in nature. An Indonesian butterfly,Papilo Palinurus, shows green on its wings, which results from thespatial mixing of structurally colored blue and yellow². Following thescheme of spatial color mixing, as shown in FIG. 3( f), a butterfly wasartificially reproduced, Papilo Palinurus by biomimetically mixingstructural colors from created by small dots of photonic crystals fixedat different colors. Magnification of the printed wing area at FIG. 3(f) shows different color dots, and each of which is the size of16.7×16.7 μm² and well below the regular human eye's resolution so thatspatially distributed dots can be seen as a single mixed color. Spatialcolor mixing makes it possible to broaden the expression range ofstructural color. It can be appreciated that a realizable possibility ofstructural color printing with fine resolution can be achieved with thedescribed technique.

By exploiting the capability of precise color and shape patterning witha single material, producing structural colors are not limited to thefixed structure on the substrate, but can be expanded to the freefloating microstructures in a microfluidic environment as color andshape encoded particles. In the field of analytical chemistry andbioscience, multiplexed assays in microfluidic environments haveattracted much attention due to their capability for high throughputscreening for drug discovery and gene expression profiling with precisecontrollability of a small volume of reactants. Various techniques togenerate encoded particles have been reported such as semiconductorquantum dots^(36,37), metallic barcode³⁸, and dot-coded particles³⁹. Incontrast to the case of quantum dot coding where precise loading ofquantum dots of different sizes is required to produce distinct encodedparticles, encoding with the invention has the advantage of simultaneousshape and color coding in a single step by using a single material in amicrofluidic environment.

In microfluidic channels made of polydimethylsiloxane (PDMS) and PDMScoated glass substrate, by virtue of an oxygen lubricating layer,microparticles generated by free-radical photopolymerization can movealong the flow stream without being stuck to the channel walls⁴⁰. Usingthis property, various color and shape encoded particles can begenerated under distinct levels of magnetic field intensity withpatterned UV light using OFML (FIG. 4( a)-4(c)). To demonstrate theconcept, the liquid curable resin containing photonic crystals wasinjected into the microfluidic channel, and generated microparticles byin-situ photopolymerization guided by patterned UV light under differentmagnetic fields (FIGS. 4( d)-4(e)). The encoded particles are caught atthe PDMS anchors and the remnant liquid resin is washed out with PEG-DAmonomer solution. Morphologies of these structures are not restricted toregular polygonal shape, but can be designed to any desired shape asdisplayed in FIG. 4. Heterogeneous encoded particles embedded withsmaller color dots were generated by sequential UV exposure undervarious magnetic fields (FIG. 4( f)). The expression of graphical code,similar to the pattern shown in FIG. 2, is limitless due to theflexibility of controlling colors and shapes.

It can be appreciated that in accordance with an exemplary embodiment, ahigh resolution patterning of multiple structural colors by a singlematerial has been demonstrated, of which the color is magneticallytunable and lithographically fixable. The versatile material, isdeveloped by magnetically assembling superparamagentic photonic crystalsinto chain-like ordered structures in photocurable resin through thebalanced interaction of magnetically induced attractive force and therepulsive forces. A unique process for immobilization of the color ofphotonic crystals is developed by taking advantage of the instantaneousnature of the OFML system. By combining photonic crystals, curable resinand OFML technique, two important applications in pattern printing andmicroparticle encoding all based on the artificial structural color ofphotonic crystals have been demonstrated. The described approachrepresents a novel multicolor patterning technique, which producescolorful patterns conveniently from a single ink instead of using manydifferent inks for different colors. It can be appreciated that thephotonic crystals based system opens a door to the wide use ofstructural color for various potential applications including structuralcolored design materials, reflective displays, and bioanalytical assay.

Methods

Material

In accordance with an exemplary embodiment, the three phase mixture ofphotonic crystals, solvation liquid and photocurable resin is used.photonic crystals were synthesized based on previously describedprotocols²⁴, which were initially dispersed in ethanol. photoniccrystals were collected by magnetic separation, and re-dispersed inphotocurable resin without complete desiccation of ethanol. Remnantethanol is used as a solvation liquid. In accordance with an embodiment,poly(ethylene glycol) diacrylate (PEG-DA, Sigma-Aldrich, M_(n)=258) with5 wt % of photoinitiator (2,2-dimethoxy-2-phenylacetophenone,Sigma-Aldrich) as the photocurable resin was used. It can be appreciatedthat other photocurable resins can include ethoxylatedtrimethylolpropane triacrylate (ETPTA), various molecular weights (Mw:258, 575, 700) of PEG-DA, 2-hydroxyethyl methacrylate (HEMA),methylmethacrylate (MMA), acrylamide (AAm), allyamine (AM) andcombinations thereof or any other material capable of being convertedfrom liquid to solid by exposure to energy of certain wavelengths.Alternatively, any material capable of being converted from liquid tosolid by exposure to temperature, energy, or other factors can be used.

Mixture of photonic crystals and photocurable resin were vortexed for 5min. For structural color printing, slide glass coated with PEG layerwas made by depositing poly(ethylene glycol) diacrylate (PEG-DA,Sigma-Aldrigh, M_(n)=258) with 5 wt % of photoinitiator(2,2-dimethoxy-2-phenylacetophenone), and photopolymerize with UV light.For particle encoding, microfluidic channel was generated using themethod based on standard soft lithography. Microfluidic channel with theheight of 40 μm was used.

Immobilization Setup

In accordance with an exemplary embodiment, a NdFeB (Neodymium IronBoron) permanent magnet was used to generate magnetic field which wasattached to the vertical stage at the microscope. For the dynamiccontrolling of magnetic field, an electromagnet coupled to the voltagecontroller was used. The photopolymerization setup used in this work wasbased on the optofluidic maskless lithography system³¹. Exposure patternof UV light was controlled by digital micromirror array (DMD, TexasInstrument) synchronized with the electromagnet, pattern of DMD and UVexposure.

Optical Characterization

Optical micrographs were acquired by true-color charge coupled device(CCD) camera (DP71, Olympus) which is directly aligned to the invertedmicroscope (IX71, Olympus). Spectrum data was acquired by spectrometer(Acton, Princeton Instrument) which is connected to the invertedmicroscope (Eclipse Ti, Nikon). Built-in field stop shutter in thespectrometer was used for isolating optical signal from background noiseand other neighboring particles. FIG. 3( c) and FIG. 3( f) were obtainedwith the commercially available digital camera (IXUS 870 IS, Canon).

Magnetochromatic Microspheres

In accordance with an exemplary embodiment, a method of formingmagnetochromatic microspheres, and more particularly to a method offorming magnetochromatic microspheres by a simultaneous magneticassembly and UV curing process of an emulsion system comprised ofsuperparamagnetic Fe₃O₄@SiO₂ colloidal particles, which areself-organized into ordered structures inside emulsion droplets of UVcurable resin.

Photonic crystal materials with band gap property responsive to externalstimuli have important applications in bio- and chemical sensors, colorpaints and inks, reflective display units, optical filters and switches,and many other active optical components.⁴¹⁻⁴⁹ Colloidal crystals, whichcan be produced conveniently by self-assembling uniform colloidalparticles, have been particularly useful for making responsive photonicmaterials because active components can be incorporated into thecrystalline lattice during or after the assembly process. The majorityof research in the field therefore has been focused on tuning thephotonic properties of colloidal systems through changes in therefractive indices, lattice constants, or spatial symmetry of thecolloidal arrays upon the application of external stimuli such aschemical change, temperature variation, mechanical forces, electrical ormagnetic fields, or light.⁴⁶⁻⁶⁶ However, wide use of these systems inpractical applications is usually hampered by slow and complicatedfabrication processes, limited tunability, slow response to the externalstimuli, and difficulty of device integration.

Because the photonic band gap is highly dependent on the angle betweenthe incident light and lattice planes, an alternative route to tunablephotonic materials is to use external stimuli to change the orientationof a photonic crystal. For easy fabrication, actuation and broaderapplications, it is highly desirable that the photonic crystals can bedivided into many smaller parts whose orientation can be controlledindividually or collectively as needed by using external stimuli.Photonic crystal microspheres, or “opal balls”, have been previouslydemonstrated by Velev et al. in a number of pioneering works by usingmonodispersed silica or polystyrene beads as the buildingblocks.^(67,68) The brilliant colors associated with thesethree-dimensional periodic structures, however, can not be tuned due tolack of control over the orientation of the microspheres. Xia et al.have introduced magnetic components into a photonic microcrystal so thatits diffraction can be changed by rotating the sample using externalmagnetic fields.⁶⁹ However, it has not been demonstrated that one cansynthesize multiple copies of such micro-photonic crystals, align themsynchronically, and collectively output uniform color signals.

Accordingly, it would be desirable to have a synthetic procedure for themanufacturing of solid microspheres containing ordered structures ofphotonic crystals, which can be called magnetochromatic microspheres.Provided here in is an exemplary embodiment for the creation of suchmagnetochromatic microspheres, wherein dispersed in emulsion droplets,superparamagnetic Fe₃O₄@SiO₂ core-shell particles self-organize underthe balanced interaction of repulsive and attractive forces to formone-dimensional chains, each of which contains periodically arrangedparticles diffracting visible light and displaying field-tunable colors.In addition, it would be desirable to have a method and/or process,which utilizes UV initiated polymerization of the oligomers in theemulsion droplets to fix the periodic structures inside the microspheresand retain the diffraction property.

In accordance with an exemplary embodiment, magnetochromaticmicrospheres can be fabricated through instant assembly ofsuperparamagnetic photonic crystals inside emulsion droplets of UVcurable resin followed by an immediate UV curing process to polymerizethe droplets and fix the ordered structures. When dispersed in theliquid droplets, superparamagnetic Fe₃O₄@SiO₂ core-shell particlesself-organize under the balanced interaction of repulsive and attractiveforces to form one-dimensional chains, each of which containsperiodically arranged particles diffracting visible light and displayingfield-tunable colors. UV initiated polymerization of the oligomers ofthe resin fixes the periodic structures inside the droplet microspheresand retains the diffraction property. Because the superparamagneticchains tend to align themselves along the field direction, it is veryconvenient to control the orientation of such photonic microspheres andaccordingly, their diffractive colors, by changing the orientation ofthe crystal lattice relative to the incident light using magneticfields. The excellent stability together with the capability of faston/off switching of the diffraction by magnetic fields makes the systemsuitable for applications such as color display, signage, and sensing.In accordance with an exemplary embodiment, a display unit that hason/off bistable states can be fabricated by embedding themagnetochromatic microspheres in a matrix that can thermally switchbetween solid and liquid phases. It can be the matrix can be a paraffin,long-chain alkanes, esters, primary alcohols, non-crosslinked polymerssuch as polyethylene, poly(ethylene oxide),polyethylene-block-poly(ethylene glycol), and/or polyesters or any othermaterial capable of being reversibly converted from liquid to solid.

It can be appreciated that among potential external stimuli, a magneticfield has the benefits of contactless control, instant action, and easyintegration into electronic devices, though it has only been usedlimitedly in assembling and tuning colloidal crystals due to thecomplication of the forces that are involved.⁷⁰⁻⁷² In accordance with anexemplary embodiment, a series of magnetically tunable photonic crystalsystems have been developed through the assembly of uniformsuperparamagnetic (SPM) colloidal particles in liquid media with variouspolarities.⁷³⁻⁷⁷ It can be appreciated that in accordance with anexemplary embodiment, the assembly of such photonic crystals includesthe establishment of a balance between the magnetically induced dipolarattraction and the repulsions resulted from surface charge or otherstructural factors such as the overlap of solvation layers. This finelytuned dynamic equilibrium leads to the self-assembly of the magneticcolloids in the form of chain structures with defined internalperiodicity along the direction of external field, and also renders thesystem fast, fully reversible optical response across thevisible-near-infrared range when the external magnetic field ismanipulated.

In accordance with an exemplary embodiment, a magnetically responsivephotonic system has been developed, wherein photonic crystalmicrospheres whose orientation and consequently photonic property can beeasily controlled by using external magnetic fields. In accordance withan exemplary embodiment, the fabrication of microspheres involvesinstant assembly of photonic crystals inside emulsion droplets of UVcurable resin and then an immediate UV curing process to polymerize thedroplets and fix the ordered structures. It can be appreciated thatunlike “opal balls” whose orientation cannot be controlled, fixing ofphotonic crystals chains makes microspheres magnetically “polarized” sothat their orientation becomes fully tunable as the SPM chains alwaystend to align along the external field direction. In addition, it can beappreciated that multiple copies of photonic crystal microspheres can befabricated in a single process, and their orientation can besynchronically tuned to collectively display a uniform color. It can beappreciated that the photonic microsphere system as disclosed does notinvolve the nanoparticle assembly step, and therefore has severaladvantages. These advantages include long-term stability of opticalresponse, improved tolerance to environmental variances such as ionicstrength and solvent hydrophobicity, and greater convenience forincorporation into many liquid or solid matrices without the need ofcomplicated surface modification. For example, in accordance with anexemplary embodiment, it can be appreciated that the magnetochromaticmicrospheres can be incorporated into a matrix, which can reversiblychange between liquid and solid phases, to produce a switchable colordisplay system whose color information can be switched “on” and “off”multiple times by means of an applied magnetic field.

The synthetic procedure of magnetochromatic microspheres in accordancewith an embodiment is illustrated in FIG. 5( a). As shown in FIG. 5( a),the magnetic iron oxide or magnetite (γ-Fe₂O₃ or Fe₃O₄) SPM particlesare first coated with a thin layer of silica (i.e., a medium) to obtaingood dispersibility and certain solvation repulsion in the curable (orphotocurable) solution. It can be appreciate that besides silica,titania (titanium oxide) and some polymer such as polystyrene andpolymethylmethacrylate might be used instead after necessary surfacemodification. The thickness of the silica coating can be controlled bycontrolling the amount of silane precursors or the catalyst. Thecontrolling of the thickness of the silica can be found in our previouspublications: (1) Ge, J. and Yin. Y., “Magnetically Tunable ColloidalPhotonic Structures in Alkanol Solutions”, Adv. Mater., 2008, 20,3485-3491. (2) Yin, Y.; Lu, Y.; Sun, Y. and Xia, Y., “Silver NanowiresCan Be Directly Coated with Amorphous Silica to Generate Well-ControlledCoaxial Nanocables of Silver/Silica”, Nano Lett. 2002, 2, 427-430, whichis incorporated herein in its entirety.

The silica coated Fe₃O₄SPM particles can be dispersed in a liquid UVcurable resin preferably containing mainly polyethyleneglycol diacrylate(PEGDA) oligomers and a trace amount of photo initiator2,2-Dimethoxy-2-phenylacetophenone (DMPA). It can be appreciated thatother suitable photocurable resins can be used including but not limitedto ethoxylated trimethylolpropane triacrylate (ETPTA), and/orpolyethyleneglycol diacrylate (PEGDA) of various molecular weights(i.e., Mw: 258, 575, 700), 2-hydroxyethyl methacrylate (HEMA),methylmethacrylate (MMA), acrylamide (AAm), allyamine (AM) and/or anycombination thereof. Alternatively, any medium capable of beingconverted from liquid to solid such that ordered structures of photoniccrystals are fixed within can be used.

The Fe₃O₄/PEGDA mixture is then dispersed in a viscous non-polar solvent(or immiscible liquid) such as silicone oil or mineral oil undermechanical stirring, which leads to the formation of an emulsion. It canbe appreciated that besides silicone oil or mineral oil, the immiscibleliquid can be paraffin oil or any oil immiscible liquid with the curablesolution, and with appropriate density and inertness to polymerize.

Upon the application of an external magnetic field, the SPM particlesself-assemble into ordered structures inside the emulsion droplets whenthe magnetically induced attraction reaches a balance with repulsiveinteractions including electrostatic and solvation forces.⁷⁶ Inaccordance with an exemplary embodiment, an immediate 365-nm UVillumination quickly polymerizes the PEGDA oligomers to transform theemulsion droplets into solid polymer microspheres, and at the same timepermanently fixes the periodic SPM structures.⁷⁸ It can be appreciatedthat any suitable photolithography setup with UV light preferably in therange of approximately 240 nm (DUV) to 365 nm (1-Line) can be used withthis system to fix the photonic structures in the resin (typical aligneror stepper). In addition, traditional mask-defined beam patterningusually requires mechanical movement of the physical mask so that anyalignment error is inevitably incorporated. However, theMaskless-Lithography proposed has the capability of high resolutionpatterning over the lithography with the physical photomasks.

In accordance with an exemplary embodiment, microspheres with differentcolors can be obtained by controlling the periodicity of the SPMassembly through the variation of the external magnetic field during theUV curing process. It can be appreciated that due to the short-rangenature of the solvation force, the range of color that can be producedfrom a single Fe₃O₄/PEGDA mixture can be limited.⁷⁶ However, inaccordance with an exemplary embodiment, in order to producemicrospheres with largely different colors such as red and blue, Fe₃O₄particles with different initial sizes or with SiO₂ coatings ofdifferent thicknesses can be used. In accordance with an exemplaryembodiment, the diameter of the microspheres typically is preferably inthe range of approximately 1 μm to 300 μm, and more preferablyapproximately 10 μm to 100 μm, depending on the type of oil and thespeed of mechanical stirring.

In accordance with an exemplary embodiment, the microspheres arepreferably large than 10 micrometer (μm), which will present aconsistent color, which is mainly contributed by the straight photonicchain structures inside the microsphere. However, it can be appreciatedthat microspheres smaller than 10 μm can be used. Once made uniformly insize, it can be appreciated that each of the microspheres should displaythe same color with magnetic tunability.

The fixation of the periodic SPM particles in the cured polymer matrixcan be verified by inspecting a section that is cut from a sample alongthe chain direction. As shown in the scanning electron microscopy (SEM)image in FIG. 5( b), parallel particle chains with regular interparticlespacing can be easily observed, providing direct support of theone-dimensional ordering of the SPM particles proposed in previousstudies.^(72,75,77) In accordance with an exemplary embodiment, sincethe cutting is not strictly along the chain direction, usually part ofthe chain is embedded inside the polymer and part of it has been peeledoff, leaving behind regular cavities. It can be appreciated theseparation between neighboring chains is typically on the order of a fewmicrometers due to the strong inter-chain repulsion induced by theexternal field.⁷⁵

The diffraction of the microspheres dispersed in a liquid can beconveniently switched between “on” and “off” states by using theexternal magnetic field, as shown in the schematic illustrations andoptical microscopy images in FIG. 5( c). In a vertical field, theparticle chains stand straight so that their diffraction is turned “on”and the corresponding color can be observed from the top. Each brightgreen dot in the optical microscopy image actually represents onevertically aligned particle chain. On the contrary, when the field isswitched horizontally, the microspheres are forced to rotate 90° to laydown the particle chains so that the diffraction is turned off andmicrospheres show the native brown color of iron oxide. It can beappreciated that the particle chains can be directly observed by carefulinspection of the microspheres through optical microscopy. The rotationof microspheres is instant, and synchronized with the manual movement ofexternal fields, as supported by the videos in the supplementaryinformation.

Depending on the direction of the external magnetic field, the particlechains can be suspended at any intermediate stage between the on/offstates with a specific tilting angle (θ). In accordance with anexemplary embodiment, the dependence of diffraction peak wavelength (λ)and intensity on the tilting angle (θ) using an optical microscopecoupled with a spectrometer is shown in FIG. 6. While the magnetic fieldis tuned within the plane constructed by the incident light and backscattered light, the diffraction from an isolated microsphere isrecorded correspondingly by the spectrometer, as schematically shown inFIG. 5( a). It can be appreciated that the diffraction peak blue-shiftswith decreasing intensity when the magnetic field direction ismanipulated away from the angular bisector of incident light and backscattered light (θ≈14.5°). FIG. 5( b) shows the spectra andcorresponding microscopy images when the angle θ is tilted from +10° to−30°. Such a change in the diffraction peak position and intensityclosely resembles the characteristics of a one-dimensional Braggphotonic crystal, as proven by the close match between the experimentalresults and theoretical simulations. Beyond −30°, the diffractionintensity is very low so that the photonic state of the microsphere canbe practically considered as “off”. FIG. 7 demonstrates the completeon/off switching of magnetochromatic microspheres that originallydiffract blue, green and red light. These microspheres are synthesizedby starting with SPM particles with average diameters of 127, 154, 197nm. It can be appreciated that by mixing of RGB (Red, Green and Blue)microspheres in various ratios can produce a great number of colors thatcan be collectively perceived by human eyes.

In accordance with an exemplary embodiment, the average size of themicrospheres can be controlled using the simple dispersing processthrough the choices of the oil type and the speed of mechanicalstirring. It can be appreciated that several methods including thoseusing microfluidic devices are available to produce monodispersedmicrodroplets.⁷⁹⁻⁸³ In general, using high speed stirring and viscousoils leads to the formation of smaller emulsion droplets. Themicrospheres prepared in mineral oils have average diameters above 50μm, and those prepared in silicone oils have average diameters less than30 μm.

FIG. 8( a) shows a series of dark-field optical microscopy images ofdifferently sized microspheres selected from the samples made by usingthe same Fe₃O₄/PEGDA mixture but with either mineral oil or silicone oilas the continuous phase. Vertical external fields are applied so thatthese microspheres are all at the “on” state. Microspheres larger than10 μm containing particle chains with spacing such that they reflect redlight all display the expected red color, which comes from thediffraction of a plurality of vertically aligned particle chains. Brightred dots, which contribute to the overall production of red color, canbe clearly observed inside the microspheres when they are imaged athigher magnification. However, in the case of microspheres 10 μm andsmaller containing similarly spaced particle chains, fewer red dots canbe observed in the center. Instead, contribution of the diffraction fromthe edge to the overall color of the microspheres gradually increases,with a progressive blue-shift from orange to yellow and eventuallyyellow-green as the microsphere size is reduced. This phenomenon can beexplained by the unique self-assembly behavior of SPM particles in thePEGDA droplets.

FIGS. 8( b)-(d) show the top-view and side-view SEM images of thetypical microspheres, suggesting that the SPM particle chains are notonly embedded inside the microspheres in the form of straight stringsbut also laid on the curved surface along the longitudinal direction.The “bent” assembly of SPM particles on the microsphere surface can beattributed to the combined effect of the spherical confinement of theemulsion droplets and the magnetically induced strong repulsive forceperpendicular to the direction of the external field. The bent surfaceassemblies can be viewed as chains tilted from the vertical directionwith the degree of tilting determined by the curvature of themicrospheres. As the microspheres become smaller, the curvature becomeslarger and the titling angle increases, leading to a blue-shift of thediffraction. Additionally, the higher surface to volume ratio of smallermicrospheres may also increase the ratio of surface chains to embeddedones and eventually change the overall diffracted color of the spheres.For microspheres larger than 10 μm, the embedded straight assembliesdominate and the bending of the surface assemblies is small, so that themicrospheres show uniform colors.

The optical response of the microspheres to the external magnetic fieldwas characterized by the switching threshold of field strength andswitching frequency, which describe how strong of an external magneticfield is required to rotate the microspheres and how fast themicrospheres respond to the changes in the magnetic field, respectively.First, a low concentration of microspheres dispersed in a densitymatched solvent—PEGDA liquid were used to measure the switchingthreshold. The dispersion was sandwiched between two hydrophobic glassslides to avoid adhesion to the glass substrate. With increasingmagnetic field strength, the microspheres were gradually turned “on” anddigital photos were taken after approximately 5 seconds of every changein the field strength. FIG. 9 shows the statistic diagrams of thepercentage of microspheres (counted in viewable area) that have beenturned “on” in an increasing field for two samples with differentloading of the magnetic materials. The corresponding accumulative curvesare also plotted from the diagrams. It has been found that the loadingof the magnetic materials in the microspheres, and not the sphere sizeis one of the factors, which determines the switching threshold of thefield strength. For microspheres with low magnetic loading (8 mgFe₃O₄/mL PEGDA), 80% of them can be turned on in a magnetic field ofapproximately 180 Gauss; while for microspheres containing more SPMparticles (16 mg Fe₃O₄/mL PEGDA), only a 100 Gauss magnetic field isrequired to turn on the same number of spheres.

The switching of diffraction could be accomplished rapidly (i.e., lessthan approximately 1 second (<1 s)) in a sufficiently strong magneticfield. Turning frequency of the microspheres was measured with a testplatform built with a halogen light source, a spectrometer and arotating magnet unit with geared DC motor. The rotating plate with NSand SN magnets standing alternately will produce a periodical vertical(1100-1200 Gauss) and horizontal magnetic field (300-400 Gauss), whosefrequency can be simply controlled by the rotating speed of the plate.

FIG. 10 shows the diffraction of microspheres in a 1.22 and 3.33 Hzvertical/horizontal alternating magnetic field, demonstrating that thephotonic microspheres can be rotated quickly. It can be noted that therotating amplitude gradually decreases with the increase of turningfrequency, primarily due to the relatively weak horizontal fieldstrength. In addition, it can be appreciated that when the frequency ishigher than approximately 7 Hz, the rotation of microspheres cannotcatch up with the external field variation so that they seem to simplyvibrate around the vertical state and the diffraction remains on all thetime. In accordance with an exemplary embodiment, the switchingfrequency can be further improved when the microspheres are dispersed ina less viscous solvent or tuned in magnetic fields with higherstrengths.

In accordance with an exemplary embodiment, the incorporation ofphotonic crystals into microspheres allows tuning of the photonicproperty by simply controlling the sphere orientation, making it veryconvenient to create bistable states that are required for a pluralityof applications such as displays. For example, a simple switchable colordisplay system in which the color information can be re-written multipletimes by means of the magnetic field. The basic idea is to createbistable states by embedding the microspheres into a matrix that can beswitched between liquid and solid states.

In accordance with an exemplary embodiment, long chain hydrocarbons andshort chain polymers, such as paraffin and poly(ethylene glycol), havemelting points slightly above room temperature. When heated, the matrixmaterial melts, allowing the display of colors by aligning themicrospheres using magnetic fields. When the system is cooled to roomtemperature, the matrix solidifies and the orientation of microspheresis frozen so that the color information remains for long time withoutthe need of additional energy. It can be appreciated that an externalmagnetic field can not alter their color once the orientation ofmicrospheres is fixed by the matrix. Reheating the matrix materials,however, will erase the particular color by randomizing the orientationof the microspheres or by magnetically reorienting the microspheres to acompletely “off” state.

FIG. 11 shows three examples of such displays fabricated by embeddingthe microspheres in polyethylene glycol (PEG, Mw=1500) films, which canbe melted at approximately 46° C. The comparison of digital photos andreflection spectra clearly demonstrates two stable diffractive states atroom temperature, suggesting the possible applications of such systemsas economical and rewriteable color display units.

It can be appreciated that the magnetochromatic microspheres can beprepared through a simultaneous magnetic assembly and UV curing processin an emulsion system. In accordance with an exemplary embodiment,superparamagnetic Fe₃O₄@SiO₂ colloidal particles are self-organized intoordered structures inside emulsion droplets of UV curable resin,followed by an immediate UV curing process to polymerize the dropletsand fix the ordered structures. In addition, it can be appreciated thatby rotating the microspheres, the orientation of the magnetic chains canbe controlled, and thereby the diffractive colors. In addition, aplurality of copies of the microspheres can be produced using theprocess, and can be tuned by external fields to collectively displayuniform colors. The excellent stability, good compatibility withdispersion media, and the capability of fast on/off switching of thediffraction by magnetic fields, also make the system suitable forapplications such as color displays, signage, bio- and chemicaldetection, and magnetic field sensing.

In accordance with an exemplary embodiment, as the size of the magnetiteparticle increases, the color red shifts (or the diffraction wavelengthincreases). As the thickness of the silica coating increases, the colorred shifts (or the diffraction wavelength increases). As the magneticfield strength increases, the color blue shifts (or the diffractionwavelength decreases). However, it can be appreciated that the color orthe diffraction wavelength is determined by not only the magnetiteparticle size, the silica coating (or coating medium), and magneticfield strength, but also many other parameters such as the chemicalnature of the resin, the surface charge of the particle surface, and theadditives.

In accordance with an exemplary embodiment, the relation of the colors(Red, Green & Blue) to the three parameters (size of magnetite particle,thickness of silica coating, magnetic field strength) is as follows, asthe overall size of Fe₃O₄/SiO₂ colloids increase from about 120 nm to200 nm, the color shifts from blue to red. As the magnetic fieldstrength increase, the color would blue shift. In accordance with anexemplary embodiment, the magnetic field preferably is in the range ofapproximately 100 Gauss to approximately 400 Gauss. It can also beappreciated that as the amount of magnetic content within a composite,which is defined as magnetic density, the more magnetic content (Fe₃O₄),less magnetic field is required to rotate the microspheres.

In the accordance with an exemplary embodiment, the method and systemsas disclosed herein, microspheres can be incorporated into a displaydevice wherein very small quanta of microspheres can be locallymanipulated to change color or to create on-off color using anintegrated micromagnetic actuator to produce local magnetic flux in thearea from several to tens of micrometers. For example, exemplary methodsand devices for actuating microspheres includes those described in ChongH. Ahn and Mark G. Allen, A Fully Integrated Micromagnetic Actuator WithA Multilevel Meander Magnetic Core, in “Solid-State Sensor and ActuatorWorkshop, 1992. 5th Technical Digest., IEEE”, 1992, page 14-18; and YaeYeong Park; Han, S. H.; Allen, M. G., Batch-fabricated microinductorswith electroplated magnetically anisotropic and laminated alloy cores,IEEE Transactions on Magnetics, 1999, 35, 4291-4300; (3) J. Park, S.Han, W. Taylor, and M. Allen, “Fully integrated micromachined inductorswith electroplated anisotropic magnetic cores,” in IEEE 13th AppliedPower Electron. Conf. Anaheim, Calif., 1998, which are incorporatedherein in their entirety disclose examples of microscale devices.

In accordance with another embodiment, it can be appreciated that theordered structures in the micromagnetospheres are composed of parallel1D chains of magnetite crystals, their spacing determined by the balanceof the attractive and repulsive forces, which in turn are affected bythe external magnetic field. In addition, it can be appreciated that thecolors exhibited by the magnetite crystals in solution, or fixed, arecreated by the ordered structures described above (1D chains).

It will be understood that the foregoing description is of the preferredembodiments, and is, therefore, merely representative of the article andmethods of manufacturing the same. It can be appreciated that manyvariations and modifications of the different embodiments in light ofthe above teachings will be readily apparent to those skilled in theart. Accordingly, the exemplary embodiments, as well as alternativeembodiments, may be made without departing from the spirit and scope ofthe articles and methods as set forth in the attached claims.

REFERENCES

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1. A method of creating colored materials, comprising: fixing orderedstructures of magnetically responsive nanoparticles within a media, suchthat the ordered structures diffract light to create colors.
 2. Themethod of claim 1, further comprising creating the ordered structures ofmagnetically responsive nanoparticles with an external magnetic field.3. The method of claim 2, wherein the ordered structures of magneticallyresponsive nanoparticles are created in a liquid media and the orderedstructures are fixed by converting the liquid media to a solid media. 4.The method of claim 3, wherein the liquid media is a photocurablesolution.
 5. The method of claim 4, further comprising fixing theordered structures of magnetically responsive nanoparticles with an UVsource having a wavelength of approximately 240 nm to approximately 365nm.
 6. The method of claim 1, wherein the ordered structures are createdin a reversible media, wherein the reversible media is reversible from asolid to a liquid, such that the color can be changed.
 7. A method ofgenerating multicolored patterns comprising: fixing a structural colorfrom a superparamagnetic colloidal nanocrystal clusters (CNC); andintroducing a high resolution patterning of multiple structural colorsusing a single material.
 8. The method of claim 7, further comprisingrepetitive tuning and fixing of the structural color from a mixture ofsuperparamagnetic photonic crystals and photocurable resin.
 9. Themethod of claim 7, wherein the superparamagnetic photonic crystalsconsists of a plurality of domain magnetite nanoparticles, which arecoated.
 10. The method of claim 7, further comprising adding an externalmagnetic field to the photonic crystals, and wherein the externalmagnetic field assembles the photonic crystals in a chain-likestructures along the magnetic field lines.
 11. The method of claim 7,wherein the attractive magnetic force due to the superparamagnetic coreis balanced with repulsive solvation force, both of which determine theinter-particle distance under any given magnetic field strength.
 12. Themethod of claim 7, wherein the inter-particle distance in a chaindetermines the color of the light diffracted from the chain.
 13. Themethod of claim 7, wherein the color can be tuned by simply varying theinter-particle distance using external magnetic fields.
 14. The methodof claim 7, wherein once the desired color is obtained, the desiredcolor is fixed by solidifying the photocurable resin through UVexposure.
 15. The method of claim 7, wherein the particle chains arefrozen in the solidified polymer network without distorting its periodicarrangements, thus retaining the structural color.
 16. The method ofclaim 7, further comprising adding a hydrogen bonding solvent, whichforms a solvation layer around the particle surface, and which providesa strong repulsion when two solvation layers overlap.
 17. The method ofclaim 16, wherein the hydrogen bonding solvent is an alkanol.
 18. Themethod of claim 17, further comprising adding ethanol to the system. 19.(canceled)
 20. (canceled)
 21. (canceled)
 22. (canceled)
 23. (canceled)24. (canceled)
 25. (canceled)
 26. (canceled)
 27. (canceled) 28.(canceled)
 29. (canceled)
 30. (canceled)
 31. A method of formingmagnetochromatic microspheres comprising: coating a plurality ofmagnetite nanocrystals with a surface medium; dispersing the pluralityof coated magnetite nanocrystals in a curable solution; placing themagnetite nanocrystals and curable solution in an immiscible solution toform an emulsion; exposing the emulsion to an external magnetic field,which aligns the coated magnetite nanocrystals in one-dimensional chainswithin emulsion droplets within the curable solution; and curing theemulsion droplets within the curable solution into magnetochromaticmicrospheres.
 32. The method of claim 31, wherein the step of curing theemulsion droplets is by exposing the curable solution to a UVillumination source.
 33. The method of claim 32, wherein the step ofexposing the curable solution to the UV illumination source fixes theordered structures within the microspheres.
 34. The method of claim 31,wherein the plurality of magnetite nanocrystals have a chemicalcomposition of γ-Fe₂O₃Fe₂O₃ and/or Fe₃O₄.
 35. (canceled)
 36. (canceled)37. (canceled)
 38. (canceled)
 39. The method of claim 31, furthercomprising microspheres immersed in a phase-changeable matrix, thephase-changeable matrix having a liquid phase and a solid phase.
 40. Themethod of claim 39, wherein when the matrix is the liquid phase,adjusting an angle of the external magnetic field to change anorientation of the microspheres.
 41. (canceled)
 42. (canceled) 43.(canceled)
 44. (canceled)
 45. (canceled)
 46. (canceled)
 47. The methodof claim 31, wherein the immiscible liquid is a viscous non-polarsolvent, mineral oil and/or silicone oil and/or paraffin oil. 48.(canceled)
 49. (canceled)
 50. (canceled)
 51. (canceled)
 52. The methodof claim 39, wherein the phase-changeable matrix is a polyethyleneglycol (PEG) film, paraffin, long-chain alkanes, esters, primaryalcohols and/or a non-crosslinked polymers such as polyethylene,poly(ethylene oxide), polyethylene-block-poly(ethylene glycol) and/orpolyesters.
 53. (canceled)
 54. (canceled)
 55. (canceled)
 56. (canceled)57. (canceled)
 58. (canceled)
 59. A method of forming magnetochromaticmicrospheres comprising: a simultaneous magnetic assembly and UV curingprocess of an emulsion system comprised of superparamagnetic Fe₃O₄@SiO₂colloidal particles, which are self-organized into ordered structuresinside emulsion droplets of UV curable resin.
 60. The method of claim59, wherein the ordered structures are fixed by an immediate UV curingprocess to polymerize the droplets.
 61. The method of claim 59, furthercomprising rotating the microspheres using an external magnetic field tochange the orientation of the magnetic chains and thereby thediffractive colors of the microspheres.
 62. A display comprising:microspheres containing ordered structures of photonic crystals, whichare rotated, which changes the angle of diffraction of light passingthrough the microspheres.
 63. The display of claim 62, furthercomprising rotating the microspheres, which changes the angle ofdiffraction of light passing through the microspheres, which changes afirst color to a second color.
 64. The display of claim 62, furthercomprising microspheres immersed in a phase-changeable matrix, thephase-changeable matrix having a liquid phase and a solid phase.
 65. Thedisplay of claim 64, wherein when the matrix is the liquid phase,adjusting an angle of the external magnetic field to change anorientation of the microspheres.
 66. The display of claim 65, whereinthe orientation of the microspheres are fixed when the matrix goes tothe solid phase.
 67. The display of claim 62, wherein the field strengthrequired to rotate the microspheres is dependent on an amount magnetitenanocrystals in each of the microspheres.
 68. The display of claim 64,wherein the phase-changeable matrix is a polyethylene glycol (PEG) film.69. The display of claim 64, wherein the phase-changeable matrix isparaffin, long-chain alkanes, esters, primary alcohols and/or anon-crosslinked polymers such as polyethylene, poly(ethylene oxide),polyethylene-block-poly(ethylene glycol) and/or polyesters.